Sliding radial key support for LNG ship tanks

ABSTRACT

In a ship having a cargo tank, with a circular horizontal crosssection, which expands and contracts in service, supported therein, the improved tank support system comprising a plurality of spaced-apart support keys mounted on and located around the periphery of the tank in a substantially horizontal arrangement, each support key having a pair of opposing spaced-apart vertical key contact faces positioned to lie in a vertical plane radial to the vertical axis of the tank, a plurality of spaced-apart support keyways joined by connecting means to a ship hold, said support keyways being located around the periphery of the tank with a support keyway opposite each support key, each support keyway having a pair of opposing spaced-apart vertical keyway contact faces positioned at about the same radial angle as the key contact faces and positioned to have a keyway contact face in mating contact with a key contact face, a plurality of load bearing surfaces on the ship hold around the periphery of the tank, and a plurality of load applying surfaces on and around the periphery of the tank in slidable pressure contact with the load bearing surfaces, whereby upon expansion and contraction of the tank with temperature change uniform slidable support contact is maintained between mating key contact and keyway contact faces, and between the load bearing and load applying surfaces.

United States Patent 11 1 Miller et a1.

1451 Sept. 30, 1975 SLIDING RADIAL KEY SUPPORT FOR LNG SHIP TANKS [75] Inventors: Clarence D. Miller, Naperville; Paul R. Johnson, Oak Lawn, both of I11.

[73] Assignee: Chicago Bridge & Iron Company,

Oak Brook, 11].

[22] Filed: Nov. 22, 1974 1211 Appl. No.: 526,131

[52] US. Cl. 114/74 R; 248/D1G. 1 [51] Int. Cl. B63B 25/08 [58] Field of Search 114/74; 220/9 LC; 248/DIG. 1, 146; 62/55, 45

[56] References Cited UNITED STATES PATENTS [34,084 6/1927 Ruths 114/74 A 3,280,778 10/1966 Leathard.... 114/74 A 3,583,351 6/1971 Gorn1an..... 114/74 A 3,583,352 6/1971 Alleaume... 114/74 A 3,841,253 10/1974 Kircik et al. 114/74 R 3,853,240 12/1974 Alleaume 220/9 LG X FOREIGN PATENTS OR APPLICATIONS 1,050,532 10/1954 Germany 248/146 Primary E.\'uminer-Trygve M. Blix Assistant Evamirrer-Sherman D. Basinger Attorney, Agent, or Firm-Merriam, Marshall, Shapiro & Klose 5 7 I ABSTRACT In a ship having a cargo tank, with a circular horizontal cross-section, which expands and contracts in service, supported therein, the improved tank support system comprising a plurality of spaced-apart support keys mounted on and located around the periphery of the tank in a substantially horizontal arrangement, each support key having a pair of opposing spacedapart vertical key contact faces positioned to lie in a vertical plane radial to the vertical axis of the tank, a plurality of spaced-apart support keyways joined by connecting means to a ship hold, said support keyways being located around the periphery of the tank with a support keyway opposite each support key, each support keyway having a pair of opposing spaced-apart vertical keyway contact faces positioned at about the same radial angle as the key contact faces and positioned to have a keyway contact face in mating contact with a key contact face, a plurality of load bearing surfaces on the ship hold around the periphery of the tank, and a plurality of load applying surfaces on and around the periphery of the tank in slidable pressure contact with the load bearing surfaces, whereby upon expansion and contraction of the tank with temperature change uniform slidable support contact is maintained between mating key contact and keyway contact faces. and between the load bearing and load applying surfaces.

8 Claims, 15 Drawing Figures U.S. Patent Sept. 30,1975 Sheet 2 of5 3,908,574

US. Patent Sept. 30,1975 Sheet 3 of5 3,908,574

US. Pamm Sept. 30,1975 Sheet4 of5 3,908,574

U.S. Patent Sept. 30,1975 Sheet 5 of5 3,908,574

SLIDING RADIAL KEY SUPPORT FOR LNG SHIP TANKS I This invention relates to cargo ships. More particularly, this invention is concerned with a support system for a storage tank in a ship which securely holds the storage tank in position yet permits horizontal and vertical dimensional change of the tank with change in temperature, such as due to a product stored therein at a temperature substantially above or below ambient temperature.

Cargo ships have been used for many years to trans port liquid materials, and particularly oil, gasoline and other such products. Until recently, most of the liquid materials have been transported in ships at ambient temperature. The support systems used for ship tanks for transporting a liquid at ambient temperature did not present a difficult problem since no unreasonable dimensional changes resulted in either the tanks or the ship hull.

Within the last few years or so it has become quite common to transport refrigerated liquid cargoes in tanks mounted in ships. The tanks used for transporting refrigerated liquid cargoes contract during initial filling when the tank temperature is lowered from ambient to the refrigeration temperature of the product during transport and expand when the tanks are empty and are allowed to return to ambient temperature. The contraction and expansion of the tank must be accommodated to prevent stresses from developing which lead to failure of the tank or the surrounding ship structure. Not only must the contraction and expansion of the tank be accommodated but, in addition, the tank must be firmly supported in the ship so that it is not damaged through the action of sea conditions which a ship can be expected to meet during a voyage. The support system must be capable of transmitting the horizontal (parallel to the ship deck) and vertical (normal to the ship deck) forces to the ship structure. It is also important for some uses that the tank support system insulate against undue heat transfer from the ship to the tank.

According to the present invention there is provided an improved tank support system for mounting a tank in a ship so that the tank can freely contract and expand through the dimensional range induced by temperature changes in the tank, such as when the tank is at ambient temperature and then is filled with a refrigerated liquid, such as a cryogenic liquid like liquefied natural gas, or with a heated liquid, for transport to a destination. The invention provides improvements in a ship having a cargo tank, particularly a cargo tank which has a circular horizontal cross-section such as is present in cargo tanks which are substantially spherical or which have a cylindrical wall and a flat, spherical or an elliptical bottom and/or top. The shape of such tanks is considered to render them particularly useful for transporting a liquefied gas or other refrigerated liquid, or even heated liquid, in a ship.

The invention furthermore is concerned with an improved tank support system which, in part, employs a plurality of spaced-apart support keys mounted on and located around the periphery of a tank in a substan tially horizontal arrangement, with each support key having a pair of opposing spaced-apart vertical key contact faces. Each of the spaced'apart vertical key contact faces is positioned to lie in a vertical plane radial to the vertical axis of the tank. In cooperation with the support keys, the system employs a plurality of spaced-apart support keyways joined by connecting means to a ship hold, said support keyways being located around the periphery of the tank with a support keyway mated to each support key. Each support keyway has a pair of opposing spaced-apart vertical keyway contact faces positioned parallel to the key contact faces so as to place an adjoining keyway contact face in mating contact with an adjoining key contact face, whereby upon expansion and contraction of the tank with temperature change parallel sliding contact is maintained between mating key contact and keyway contact faces, thereby maintaining the tank in position against horizontal forces applied to the tank.

The improved tank support system also includes a plurality of load bearing surfaces on the ship hold around the periphery of the tank and a plurality of load applying surfaces on and around the periphery of the tank in slidable, pressure contact with the load bearing surfaces. The weight of the tank and the product load are thereby transferred by the load applying surfaces to the load bearing surfaces. This transfer of the gravity load, according to the invention, is effected even while sliding movement between the surfaces takes place with expansion and contraction of the tank induced by temperature change.

The load bearing and load applying surfaces are advisably horizontal. Furthermore, the load applying and load bearing surfaces are generally best positioned between a pair of opposing spaced-apart vertical key contact faces.

The tank support system also advisably includes an antifioatation means in slidable contact with the tank to keep the tank in position if the hold partially floods or has upward lift due to the tank contents at any one time.

The tank support system also is beneficially provided with thermal insulation to retard heat flow from the ship through the tank support system to the tank. The load bearing and load applying surfaces, and the key contact and support keyway contact faces, can be metallic or nonmetallic, provided of course that suitable insulation is used-in conjunction with metallic faces and surfaces to retard heat flow to the tank.

A ship cargo tank is generally mounted so that at least a substantial part of it is located below deck in a ship hold. To retard heat leak between a refrigerated liquid product in the tank and the ship hold it is conventional to insulate either the tank wall or the ship hold wall. The subject invention can be employed with either of these insulation systems.

The invention will be discussed further in conjunction with the attached drawings, in which:

FIG. 1 is an isometric view of a ship at sea containing five spherical tanks; using the support system of this invention, for transporting liquefied natural gas;

FIG. 2 is a lateral vertical cross-sectional view through the ship of FIG. 1, and shows one embodiment of a support system for the tank;

FIG. 3 is an enlarged vertical sectional view through a support key and cooperating support keyway of the support system used with the tank of FIG. 2;

FIG. 4 is a front view of the cooperating support key and support keyway shown in FIG. 3;

FIG. 5 is a plan view, partially schematic, showing the radially positioned support key faces and radially positioned faces on the support keyway;

FIG. 6 illustrates a front view of a second embodiment of a support key and cooperating support keyway within the invention;

FIG. 8 is a front view of a third embodiment of support key and cooperating support keyway for supporting a tank in a ship hold;

FIG. 9 is a sectional view taken along the line 99 of FIG. 8;

FIG. 10 is a front elevational view, partially in section, of a fourth embodiment of the invention;

FIG. 11 is a sectional view taken along the line 1111 of FIG. 10;

FIG. 12 is a front elevational view of another embodiment of the invention;

FIG. 13 is a side elevational view of still another embodiment of the invention;

FIG. 14 is an elevational sectional view taken along the line 14--14 of FIG. 13; and

FIG. 15 is a plan sectional view taken along the line 15-15 of FIG. 14.

So far as is practical, the same elements or parts which appear in the different views of the drawings, and embodiments of the invention, will be identified by the same numbers.

The ship 10 shown in FIG. 1 contains five spherical tanks 11 for transporting liquefied natural gas. As shown in FIG. 2, each tank 11 has a spherical metal tank shell 12 located in the hold of ship 10. The wall 13 of the hold is covered by insulation 14 (FIG. 3) and the layer of insulation is covered by a spray shield 15. A weather shield 16 extends upwardly from the ship deck over that part of the tank shell 12 which extends above the ship deck to thereby protect the tank against adverse weather conditions. The weather shield 16 includes insulation to retard heat leak to an acceptable limit. Access cell 17 is provided at the top of the tank shell 12 for piping and controls for use in filling and emptying the tank.

With reference to FIGS. 2 to 4 of the drawings, ring plate 21 is horizontally positioned on, and extends around, tank shell 12 advisably at or beneath its equator. Projecting downwardly from ring plate 21 are vertical support plates 22 and 23. The vertical support plates 22 and 23 terminate at support key 24 and are joined therto by welding. Keys 24 are equally positioned around tank shell 12 in a horizontal plane. The support keys 24 are advisably made of metal. The vertical support plates 22 and 23 are held in spaced-apart relationship by horizontal plate 25. Vertical plates 26 extend downwardly from horizontal plate and are welded at their lower ends to the top of support key 24. Partially beneath support key 24 is a support keyway 30. Support keyway 30 has, in part, a base of load supporting material 31, such as a block of compressed wood impregnated with a polymeric material. On top of the base 31 is positioned a horizontal metal plate 32 which is in contact with the bottom surface of key support 24. Beneath the base 31 is positioned a metal plate 33 which is securely mounted on a supporting shelf of the hold wall 13. The interface between plate 32 and the bottom of support key 24 provides load applying and load bearing surfaces by which the weight of the tank and its contents is transferred to the ship. Furthermore, slidable movement of the bottom face of support key 24 with respect to the top of plate 32 is readily FIG. 7 is a sectional view along the line 7-7 of FIG.

achieved automatically upon expansion and contraction of tank shell 12 with temperature change.

The support key 24 has a pair of opposing vertically positioned contact faces 35 on each end or opposite side thereof and each contact face 35 is positioned radial to a vertical plane which passes through the tank vertical centerline as is more clearly shown in FIG. 5.

Each of the support keyways 30 has opposing vertical contact faces 36 in contact with the vertical Contact faces 35 of support key 24. The support keyway opposing contact faces 36 are the outer surfaces of metal plates 37 which are joined to load bearing and insulating nonmetallic blocks 38, which in turn are supported by nonmetallic load bearing and insulating blocks 39. Blocks 38 and 39 can be made of compressed wood impregnated with a polymeric material.

To hold adjacent support keyways 30 in spaced-apart position spacer 40 is positioned between them. Hollow elements 41 are positioned between the sides of the spacer 40 and the blocks 38. Insulation 43 is placed on the top of spacer 40 and on the top of each support keyway 30 and is extended to the sides of adjacent support key 24.

Horizontal uncontrolled movement of the tank shell 12 is prevented by having the key contact faces 35 in contact with the support keyway faces 36. The contact between the faces is maintained even though the tank shell 12 increases and decreases in diameter with temperature change because the mating support key and support keyway faces are radially positioned with respect to the tank shell. This permits slidable contact between the mating faces when temperature induced expansion and contraction of the tank shell results.

Upward displacement of tank shell 12 is restrained by a series of bolts 47 which extend from a flange 48 extending inwardly from the top part of key 24. The lower end of each bolt 47 is joined to a flange 49 attached to the ship hold wall 13 and surrounded by insulation. Approximately four bolts 47 are joined in such arrangement with each key 24.

FIGS. 6 and 7 show a second embodiment of a support system according to the invention l-lorizontally positioned ring plate 51 extends around a tank shell 12 substantially below the tank equator. In addition, a second horizontally positioned ring plate 52 also extends around the tank shell 12 but spaced below ring plate 51. Vertical plates 53 extend between the ring plates 5 1 and 52 and are joined thereto. Attached to the bottom of ring plate 52 is a series of support keys 54. Each support key 54 is joined to ring plate 52 by bolts 55.

Support keyway 56 is joined to the hold wall 13 of the ship. The Support keyway 56 has a metal base with an upwardly projecting rectangular central portion 57. On each side of the central portion 57 is positioned a nonmetallic load bearing and insulating block 58 with a block 59 of similar material positioned on top thereof. The blocks 58 and 59 are firmly connected to the metal base 60 central portion 57 so that they remain stationary. Each support key 54 has a pair of spaced-apart vertical contact faces 61 radially positioned with respect to tank shell 12. Similarly, the blocks 58 have opposing contact faces 62 vertically and radially positioned to mate with the support key contact faces 61. Insulation 69 is positioned around the metal base 60 to retard heat transfer.

With reference to FIG. 7, bolt 63 is joined at its upper end to ring plate 51 and at its lower end to a flange 64 which is joined to the ship hold wall 13. A plurality of bolts 63 is positioned around tank shell 12 to restrain it against upward movement.

A plurality of support keys 54 is positioned equally around tank shell 12 and for each support key 54 a support keyway 56 is provided in cooperating arrangement thereto attached to the ship hold wall. In this way, the tank shell 12 is securely held in position to resist lateral forces, yet it is free to expand and contract with temperature change. Any increase or decrease in dimensions of the tank shell 12 is readily accommodated by the sliding arrangement provided between the contact faces 61 and 62. Furthermore, the load from the tank and the tank contents is transferred to the ship by having the key support 54 rest on the top of block 59 which provides a load bearing surface.

A third embodiment of the invention is shown in FIGS. 8 and 9. In this embodiment, which is somewhat similar to the embodiment shown in FIGS. 6 and 7, a plurality of load bearing heat insulating blocks 65 is joined in equally spaced-apart relationship to the bottom of the ring plate 52. Bars 66 and 67 are fixedly joined to ring plate 52 and secure each block 65 in position against radial independent movement of the block 65 and ring plate 52. Block 65 constitutes a support key and its bottom surface 68 is intended to be slidably positioned on the upper metal Surface 71 of support keyway 72. I

The support keyway 72 has a horizontal plate 73 which is securely joined to ship hold wall 13. The plate 73 has a pair of upwardly extending vertically positioned side bars 74 and 75 which contact the vertical radially positioned side faces 76 of the support key 65. The faces 77 of the bars 74 and 75 are vertically positioned and radially oriented with respect to the tank shell 12 so that they are essentially in faceto-face contact with the faces 76 of the support key 65. Triangular braces 78 help support the vertical bars 74 and 75 is position. No bars like bars 74 and 75 are placed at the ends of plate 73 so that as a result the support key 56 is free to slide on the face 71 of plate 73. The load of the tank and the tank contents is transferred to the ship hold by having the bottom surface 68 of the support key 65 in slidable contact with the face 71 of plate 73.

A fourth embodiment of the invention is illustrated by FIGS. and 11. The support key 80 shown in FIGS. 10 and 11 contains a load bearing block 81 which has low heat conductive properties. A plurality of the support keys 80 are positioned equally in a horizontal plane around the tank shell 12 and are joined to the lower surface of the ring plate 52. Metal plate 82 is positioned at the bottom of block 81. Bolts 83 are joined to the plate 82 and extend through the block 81 to be fastened to plate 52 to thereby securely hold support key 80 in fixed position. The lower side areas of block 81 are provided with vertically positioned plates 84. The vertical opposing faces of plates 85 are positioned radially with respect to the tank shell 12.

Each of the support keys 80 is cooperatively positioned with respect to a support keyway 88. Each support keyway 88 has a horizontally positioned plate 89 which is joined to the ship hold wall 13. At each of two opposing side edges of plate 89 is located a vertically positioned metal bar 90 which is held in place by triangular braces 91. The vertical inner surfaces 92 of bars 90 are radially positioned with respect to the tank shell 12 so that the faces 92 of the support keyway 88 are arranged to be face-to-face with the contact faces of the support key 80. In this way, the tank shell 12 is held in secure arrangement by the plurality of support keys and support keyways with respect to the ship hold, yet the tank shell 12 is free to expand and contract With change in temperature. The lower face 93 of plate 82 slidably transfers the tank load and the contents of the tank to the upper surface 94 of plate 89 to thereby transfer the load into the ship hold.

Still a fifth embodiment of the invention is illustrated in FIG. 12. As shown in FIG. 12, the support keyway 101 is joined to the ship hold wall 13. Included as part of the support keyway 101 is a metal base 104 which has vertical side walls 105. Load bearing blocks 106 and 107, made of a material which has good load bearing and low heat conductive properties, are positioned in the metal base 104. Mounted on top of block 107 is a cast stainless steel stool 108. Filler material 102 is placed in the space between the walls and blocks 106. Insulation 109 is placed around the support keyway 101. Support key isjoined by bolts 116 to ring plate 53 mounted on tank shell 12. The opposing vertical contact surfaces 117 of the support key 115 are radially positioned with respect to the tank shell 12 and are in approximately face-to-face contact with vertically positioned contact surfaces 118 of the stool 108. The contact faces 1 18 form part of the support keyway 101 and are also radially positioned with respect to the tank shell 12. The slidable face-to-face contact between the contact faces 117 and 118 of the cooperating support keys 115 and support keyway 101, positioned completely around the tank shell 12, prevent the tank from being displaced horizontally when lateral forces are applied thereto. The tank shell 12 is nevertheless free to slidably expand and contract with changes in temperature. Furthermore, the entire load of the tank and tank contents is transferred by means of the support keys 115 to the stool 108 of the support keyway 101. Transfer of the gravity load is through the load applying contact face 119 of the support key to the mating load bearing contact face 120 of the support keyway. The contact faces 119 and 120 are adapted to permit horizontal sliding with respect to each other so that the tank can expand and contract with temperatureinduced dimensional change.

A material having low friction properties, such as Teflon, can be used on the mating sliding faces to prevent any binding. Binding is not likely, however, since all major dimensional change in the tank shell will take place before it carries much of a load since a liquefied gas will quickly cool down the tank even though present only in a small amount.

A sixth embodiment of the invention is shown in FIGS. 13 to 15. As shown in these figures, support keyway is joined to the ship hold wall 13. The support keyway 130 has a metal, load bearing contact face 131 and opposing vertical contact surfaces 132. Supporting key 133 is joined to ring plate 134 mounted on tank shell 12. Vertical stiffeners 135 extend upwardly from ring plate 134 to ring plate 136 on tank shell 12. The support key 133 contains load bearing insulating block 137 at the bottom. The face 138 of insulating block 137 is in slidable contact with contact face 131. In addition, support key 133 contains insulating blocks 139 on each side thereof and the face 140 of insulating blocks 139 are placed in slidable contact with contact surfaces 132. A number of antifloatation chocks 145 is provided around the tank to resist upward movement of the tank if the hold should flood. Each antifloatation chock 145 consists of a vertical plate 146 joined to the ship hold wall. Load bearing insulation block 147 is mounted on the bottom of plate 146 and the lower face or surface 148 thereof is placed in slidable contact with the upper surface of support key 133. Insulation 149 and 150 is placed on the tank shell and tank supporting structure as needed to reduce heat leak and to prevent undue cooling of the ship hold wall when a cryogenic liquid is in the tank.

Any number of support keys and cooperating support keyways of the type illustrated by FIGS. 13 to 15 can be used as warranted to support a tank in a ship hold. The vertical faces of the keyways and support keys in contact with each other are radially positioned with respect to the vertical center line of the tank. In addition, any suitable number of antifloatation blocks can be used in combination with the described tank support system of this embodiment or with any of the other embodiments of the invention disclosed herein.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. In a ship having a cargo tank, with a circular horizontal cross-section, which expands and contracts in service, supported therein, the improved tank support system comprising:

a plurality of spaced-apart support keys mounted on and located around the periphery of the tank in a substantially horizontal arrangement;

each support key having a pair of opposing spacedapart vertical key contact faces, with each key contact face lying in a vertical plane radial to the vertical axis of the tank;

a plurality of spaced-apart support keyways joined by connecting means to a ship hold, said support keyways being located around the periphery of the tank with a support keyway opposite each support y;

each support keyway having a pair of opposing spaced-apart vertical keyway contact faces positioned at about the same radial angle as the key contact faces and positioned to have a keyway contact face in mating contact with a key contact face;

a plurality of load bearing surfaces on the ship hold around the periphery of the tank; and

a plurality of load applying surfaces on and around the periphery of the tank in slidable pressure contact with the load bearing surfaces;

whereby upon expansion and contraction of the tank with temperature change uniform slidable support contact is maintained between mating key contact and keyway contact faces, and between the load bearing and load applying surfaces.

2. A tank support system according to claim 1 in which the tank support system includes thermal insulation to retard heat flow from the ship through the tank support system to the tank.

3. A tank support system according to claim 1 in which the load'bearing and load applying surfaces are horizontal.

4. A tank support system according to claim 3 in which both of the load applying and load bearing surfaces are metallic.

5. A tank support system according to claim 1 in which the load applying and load bearing surfaces are between a pair of opposing spaced-apart vertical key contact faces.

6. A tank support system according to claim 1 in which at least one of the load applying and load bearing surfaces is a nonmetallic material.

7. A tank support system according to claim 1 including means to hold the load applying surface in contact with the load bearing surface against vertical uplift load on the tank.

8. A tank support system according to claim 1 in which the support keys have freedom of displacement in the radial direction of the tank but are restricted from displacement in the tangential direction.

* l l l 

1. In a ship having a cargo tank, with a circular horizontal cross-section, which expands and contracts in service, supported therein, the improved tank support system comprising: a plurality of spaced-apart support keys mounted on and located around the periphery of the tank in a substantially horizontal arrangement; each support key having a pair of opposing spaced-apart vertical key contact faces, with each key contact face lying in a vertical plane radial to the vertical axis of the tank; a plurality of spaced-apart support keyways joined by connecting means to a ship hold, said support keyways being located around the periphery of the tank with a support keyway opposite each support key; each support keyway having a pair of opposing spaced-apart vertical keyway contact faces positioned at about the same radial angle as the key contact faces and positioned to have a keyway contact face in mating contact with a key contact face; a plurality of load bearing surfaces on the ship hold around the periphery of the tank; and a plurality of load applying surfaces on and around the periphery of the tank in slidable pressure contact with the load bearing surfaces; whereby upon expansion and contraction of the tank with temperature change uniform slidable support contact is maintained between mating key contact and keyway contact faces, and between the load bearing and load applying surfaces.
 2. A tank support system according to claim 1 in which the tank support system includes thermal insulation to retard heat flow from the ship through the tank support system to the tank.
 3. A tank support system according to claim 1 in which the load bearing and load applying surfaces are horizontal.
 4. A tank support system according to claim 3 in which both of the load applying and load bearing surfaces are metallic.
 5. A tank support system according to claim 1 in which the load applying and load bearing surfaces are between a pair of opposing spaced-apart vertical key contact faces.
 6. A tank support system according to claim 1 in which at least one of the load applying and load bearing surfaces is a nonmetallic material.
 7. A tank support system according to claim 1 including means to hold the load applying surface in contact with the load bearing surface against vertical uplift load on the tank.
 8. A tank support system according to claim 1 in which the support keys have freedom of displacement in the radial direction of the tank but are restricted from displacement in the tangential direction. 